Patina finish on cupreous surfaces



3,473,970 PATlNA FliNlSl-I N CUPREOUS SURFACES Paul Rohey, Brooklyn, N.Y., assignor to International Copper Research Association, Inc, New York, N.Y., a corporation of New York No Drawing. lFiled June 13, 1966, Ser. No. 556,909 Int. Cl. C23f 7/08; C23c 1/10 iU.S. C1. 148-6.21 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a patinating composition and, more particularly, to a coating composition and a method for imparting a decorative patina finish to a cupreous surface.

Normally, when copper and its alloys are exposed to the natural atmosphere, their surface colors will gradually change according to the copper composition and the atmospheric conditions. In the natural Weathering of copper, the color changes gradually from the bright metallic pink of the new metal through red brown, dark brown, ebony, gray and gray green to the ultimate blue-green patina. The color change of some copper base alloys follows an analogous pattern, though rate of change and colors produced may differ from those on unalloyed copper.

The gradual color change of the cupreous surface is due to the slow formation of a relatively thin surface layer of copper compounds produced by the chemical reaction of the cupreous surface with atmospheric constituents, such as oxygen, carbon dioxide, hydrogen sulfide, sulfur dioxide, and sodium chloride in the presence of atmospheric humidity, dew, and rain. Depending on the composition of the cupreous surface, the climate and certain atmospheric contaminants, this natural patina may appear on the surface only after exposure to the elements for decades or centuries. The colors are very pleasing in appearance and are desirable in many architectural and decorative applications. The length of time required and the necessity for certain atmospheric and climatic conditions, however, render dependence on the natural process often impractical. Therefore, many artificial methods using chemicals or electrolytic processes were proposed to provide the cupreous surface with a pleasing and stable patina finish in a short length of time.

The chemicals, or mixtures of chemicals, used in the prior methods are intended to react relatively quickly with the cupreous surface to provide a surface layer of adhering, insoluble copper compounds. The color of this surface layer may vary considerably depending on the chemicals used and the compositions of the cupreous surface. Moreover, repeated applications are necessary with many of these prior processes, in order to obtain sufficient depth of color.

Of the various methods developed, the electrolytic method of Vernon and the CABRA (The Copper and Brass Research Association in America) spray process have been considered to be the most promising methods. In the Vernon anodic method, an electrolyte consisting of mag- States Patent 0 Patented Oct. 21, 1969 "ice nesium sulfate, magnesium hydroxide, and potassium bromate in water, is used for the patinating process. The deposit obtained, however, still requires aging to fully develop the patina. Because it is an electrolytic method, patinating treatments of large surface areas become difficult if not impossible. The CABRA process requires a thoroughly clean and oxide-free surface before the application of an aqueous coloring solution consisting of ammonium sulfate, copper sulfate, and ammonia. The

amount of ammonia used must be measured exactly since the ratio of ammonia to water is critical. The solution is spray-coated on the surface and allowed to dry before a second application can be made. The spraying and drying cycle must be repeated five or six times, and often up to eight cycles are required. A properly sprayed finish appears glassy and requires weathering for the full patina to develop under suitable atmospheric conditions.

It is the object of this invention to overcome the disadvantages of the prior methods by providing a process and a coating composition which have the advantages of greater simplicity and versatility and are capable of providing a patina similar to the one produced by the natural aging process. According to the present invention, no spray or electroplating equipment is necessary because the solution can be applied advantageously by brush. Furthermore, only a single application is required.

Broadly stated, the coating composition of this invention comprises an aqueous acidic solution in which there are incorporated copper sulfate, ammonium sulfate, an oxidizing agent, a humectant, and a thickener. The amount of each ingredient to be used in the composition can be varied within a wide range depending on the composition of the cupreous surface, the desired patina finish, local atmospheric conditions and other variables. Generally the amount of thickener to be used should be suflicient to improve the rheology of the composition so that it can be applied in a sufficiently heavy coat without sagging or running down the roof.

I found by using a composition of this invention a pleasing patina finish can be imparted to cupreous surfaces without the necessity of repeated application required by prior methods. While the exact mechanism of patina formation using the composition of this invention is not certain, the use of the oxidizing agent appears to reduce the dependence of the composition on atmospheric oxygen required for patina formation and the humectant prevents rapid loss of moisture in the applied film thus promoting continuous reaction between the chemicals and the cupreous substrate over a period of hours. The most suitable oxidizing agents are sodium, potassium or ammonium dichromate. The preferred humectant is lithium chloride.

A thickening agent suitable for the present invention should be compatible with the other ingredients without being salted out or chemically precipitated, and should have a strong thickening action with water. A specially processed magnesium montmorillonite known under the trade name Ben-A-Gel and a polysacharide gum known as Kelzan have been found to be eminently suitable.

The coating compositions is rendered acidic just prior to use by the addition of an acid such as hydrochloric or acetic acid. The patinating solution, after acid addition, should be used within 24 hours. The stock solution without the acid is stable indefinitely.

In order to further illustrate this invention, specific examples are described below. In these examples some of the ingredients are present in relatively high concentrationsnear the saturation point. This is particularly true of the copper sulfate and ammonium sulfate. However, none of the concentrations is critical and the amount of any ingredient may be raised or lowered by about 15% (solubility permitting) without drastically affecting the usefulness of the solution.

The 6% Ben-A-Gel is prepared by adding 60 g. dry Ben: A-Gel EW slowlyto 940 g. of water using a high-shear disperser, e.g. a Cowles dissolver. Dispersion is continued for about minutes until a rather stiff paste is formed.

Further processing is done in a nonmetallic vessel (plastic, glass, or wood) provided with a plastic-coated electric stirrer. The 6% Ben-A-Gel paste is placed in the vessel and the water is stirred in. The' salts are dissolved in the mixture byadding them one at a time, waiting until the solution is almost complete before adding the next salt.

The patinator can be applied by brush to new or weathered copper. Weathered (oxidized) copper is preferable as it provides better adhesion for the patina. In this case, the amount of hydrochloric acid may be reduced by 20%. The surface should be cleaned with water and a detergent, but no abrasive cleaner should be used. In case the solution is applied to weathered copper, the latter should have a dark brown appearance which is assumed by copper after several months exterior exposure. Very heavily weathered, i.e., blackened copper, is less suitable as it impairs the bright appearance of the patina. The color tends to improve, however, on prolonged exposure.

Development of the patina, generally, begins immediately after application, and after several hours the surface presents a vivid green appearance. This color changes within a few weeks, on exposure to the weather, to a pale bluish-green color resembling natural patina.

In this example the composition may-be varied in accordance with the formula stated below:

Parts by wt. Copper sulfate, CuSO -5H O 85-120 Ammonium sulfate, (NH SO 50-65 Lithium chloride, LiCl 85-115 Sodium dichromate, Na Cr O 5-7 Hydrochloric acid, HCl (37%) 130-170 Magnesium montmorillonite (6%) 320-430 Water 170-230 EXAMPLE 2 7 Parts by wt. Copper sulfate, CuSO -5H O 121 Ammonium sulfate, (NH SO 67 Lithium chloride, LiCl 108 Ammonium dichromate, (NH Cr O-; 17 Acetic acid (glacial), CH COOH 34 Ben-A-Gel EW (6%) 337 Water 316 4 Similarly the composition may be varied as follows:

1 Partsbywt. Copper sulfate, CuSO -5H O 90-130 Ammonium sulfate, (NH SO 50-70 Lithium chloride, LiCl 90-125 Ammonium dichromate, (NH Cr O -20 Acetic acid (glacial), CH COOH 30-40 Magnesium montmorillonite (6%) 285-400 Water 270-365 The patina prepared in accordance with this invention remains sensitive to rain for a few days and should be applied, therefore, during a period of dry Weather. However, this initial water sensitivity can be greatly reduced by applying a protective overcoat. A low-solids polyvinyl butyral solution which has proven very suitable for this purpose has the following composition:

Parts by wt.

' Polyvinylbutyral in ethyl alcohol) 250 Cellosolve acetate 750 The overcoat may be applied by brush as soon as the patinating solution has dried thoroughly. This may be done after 5-8 hours depending on weather conditions.

I claim: 1. A coating composition for imparting a decorative patina finish to a cupreous surface, said composition having the following formulation:

Parts by wt. Copper sulfate, CuSO -5H O -120 Ammonium sulfate, (NH SO 50-65 Lithium chloride, LiCl 85-115 Sodium dichromate, Na Cr O 5-7 Hydrochloric acid, HCl (27%) -170 Magnesium montmorillonite (6%) 320-430 Water -230 2. A coating composition for imparting a decorative patina finish to a cupreous surface, said composition having the following formulation:

References Cited UNITED STATES PATENTS 1,951,304 3/1934 Freeman et al 1486.24 2,001,753 5/1935 Tanner et al. 148-6.24 X 2,054,737 9/1936 Brunner 1486.24 3,284,249 11/ 1966 Osborn 1486.24

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,473,970 October 21, 1969 Paul Robey It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, immediately following "Example 2" after the figure "1000", add the following:

-- Preparation is the same as in Example I.

Signed and sealed this 3rd day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

